1. Field of the Invention
The present invention relates generally to the fields of chemistry and antineoplastic pharmacology. More specifically, the present invention relates to competitive inhibitors of glyoxalase I and methods of generating such competitive inhibitors inside tumor cells.
2. Description of the Related Art
Recent advances in understanding methylglyoxal metabolism suggest that the glyoxalase pathway might be a reasonable target for antitumor drug development (1). The apparent physiological function of this pathway is to remove cytotoxic methylglyoxal from cells as D-lactate via the sequential action of the isomerase glyoxalase I (GlxI) and the thioester hydrolase glyoxalase II (GlxII), FIG. 1. Methylglyoxal appears to arise primarily as a by-product of the interconversion of intracellular triosephosphates, as well as from other sources (3,4). High concentrations of exogenous methylglyoxal exhibit selective inhibitory activity toward rapidly proliferating tumor cells versus nonproliferating normal cells in vitro (5-8). The molecular basis of methylglyoxal toxicity is not clearly understood, but may involve inhibition of DNA and protein synthesis (5,9). Indeed, methylglyoxal is known to form adducts with nucleic acids (10). Supporting a detoxification role for the glyoxalase pathway is the demonstration that transfected murine NIH3T3 cells that overexpress the gene for GlxI are exceptionally resistant to the cytotoxic effects of exogenous methylglyoxal (11).
Vince and Daluge suggested that inhibitors of GlxI might function as antitumor agents by inducing elevated levels of methylglyoxal in cells (12). Although numerous GSH-based competitive inhibitors have been described, a method for efficiently delivering these compounds into cells has not been available (2). Importantly, Lo and Thornalley reported that the competitive inhibitor S-p-bromobenzylglutathione can be indirectly delivered into human leukemia (HL60) cells as the [glycyl, glutamyl] diethyl ester prodrug, and that this compound is both cytostatic and cytotoxic to these cells (13). This prodrug strategy functions on the basis that after the diethyl ester diffuses into the cells, nonspecific esterases catalyze the deethylation of the diethyl ester to give the inhibitory diacid. The same laboratory reported that S-p-bromobenzylglutathione diethyl ester is toxic to a range of different human tumor cell lines in culture.
In any event, this prior art prodrug strategy is limited in at least two respects. First, the diffusion of the diethyl esters into cells is a relatively slow process. For example, the apparent first order rate constant for diffusion of 2Et).sub.2 into L1210 cells in vitro is only 0.04 min.sup.-1 at 37.degree. C. This probably reflects the fact that 2Et).sub.2 is a cationic species under physiological conditions, with limited solubility in the lipid bilayer of the cell membrane. Second, in contrast to human plasma, the plasma of most common strains of tumor-bearing laboratory mice contain high levels of plasma esterases which catalyze rapid deethylation of the diethyl ester prodrugs, T.sub.1/2 &lt;30 sec. Therefore, costly esterase-deficient tumor-bearing mice are required in order to evaluate the in vivo antitumor activities of the diethyl ester prodrugs.
The prior art is deficient in the lack of effective means of delivering competitive inhibitors of the methylglyoxal-detoxifying enzyme glyoxalase I into cells. The present invention fulfills this longstanding need and desire in the art.